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Old 28th Mar 2020, 12:04 am   #21
Boulevardier
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Default Re: Mullard 520 OP Transformer

Quote:
Originally Posted by Radio Wrangler View Post
It's a learning exercise!

Those of us who went "Oooo-Errr, it's the layout" all learned the importance of such things by getting it wrong and getting our fingers burned.

Welcome to the club

Valve amps involve high stage gains, and rather high impedances and so layout can be touchy. transistors have their own problems.

David
Indeed! And I couldn't even begin to list the mistakes I've made (and continue to make) while on that learning curve.

Have you got the original publication "Mullard Circuits for Audio Amplifiers"? It gives very detailed construction notes for the 5-20, and has some useful general introductory chapters on theory, design and construction which are well worth reading. It is available (free) from various places on the web. There are also plenty of people on here who can help with questions and details.

Mike
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Old 28th Mar 2020, 3:00 pm   #22
PJL
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Default Re: Mullard 520 OP Transformer

There are plenty of 5-20 amplifiers that do not religiously follow the Mullard layout, Leak to name a famous one. In any case, unless you are going to have steel chassis made, you will not be able to do an exact reproduction. It looks like the critical components in your layout have been arranged OK and are spaced well apart so it should be possible to vastly improve the situation with rewiring.

1. Use a busbar for earthing as in the Mullard layout and run the busbar past each valve in order, V1, V2, V3/V4. The busbar should ideally be thick tinned copper wire but 2.5/4mm solid copper wire (mains) would do.
2. Where Mullard have wired components direct to the valve holders, you should do the same.
3. Where you have used tagboards, make sure the components are placed in the same order as in the Mullard layout and the distance to the associated valve is kept to a minimum.
4. Keep the wiring to V1 away from any other wiring.
5. Wire up the heaters with lightly twisted cable in order V3/V4, V2 and V1 laying the cable close to the metal panel.
6. Only connect the busbar to the metal panel at one point as described in the Mullard manual.
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Old 29th Mar 2020, 6:03 pm   #23
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Default Re: Mullard 520 OP Transformer

PJL -Thanks for the information, all points are noted.

I've now stripped the amp of all the old wiring, and connectors.

One question I have is related to choice of interconnecting cable. Would you recommend using suitably V&I rated multi-strand cable or solid core.

Thanks

Jon
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Old 29th Mar 2020, 6:42 pm   #24
John Caswell
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Default Re: Mullard 520 OP Transformer

Hi Jon,
For general interconnects ie anodes, screens grids etc I use 14/0.2 of various colours. Your choice of PVC, PTFE, Silver plated, Gold plated, solid Platinum
Heater wiring for the EL34s and rectifiers requires something more susbstantial. I use electrical switch gear wiring something like 6491X in different colours, twisted together. I would try to wire the EL34 valve heaters in phase ie pin 2 to pin 2 as there is some evidence that it helps in hum reduction, you only need to use 14/0.2 for the EF86/ECC83 as their current consumption is relatively low. Try to arrange the heater wiring so that it does not "surround/wrap around" the valve base especially the EF86/ECC83.

John
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Old 29th Mar 2020, 9:10 pm   #25
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Default Re: Mullard 520 OP Transformer

I always prefer solid-strand wire for chassis wiring - once you've shaped/bent it, it stays where you've put it. Multistrand, being more flexible/springy by design, tends to have its own ideas about where it going to settle, especially if you have to disturb it later to fit other wiring/components. Twisted heater wiring may be better multistrand, but even solid can be carefully twisted.

Mike
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Old 29th Mar 2020, 10:13 pm   #26
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Default Re: Mullard 520 OP Transformer

Aviation grade wire with 'Kynar' insulation is nice to use. Easier to strip than straight PTFE insulation, and survives soldering temperatures.

Laced harnesses look gorgeous, but maximise cross coupling... not an easy choice.

David
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Old 29th Mar 2020, 10:30 pm   #27
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Default Re: Mullard 520 OP Transformer

Most of the gain is in V1 so it is critical to keep other connecting wires well away from V1 and the associated components.

I am trying to resolve a problem with a Pioneer A-443 solid state amp that screams when in phono (MC) and the volume is turned up. The phone amp is on a separate board that is not screened nor are the interconnecting wires back to the socket on the main board. If you wave your hand anywhere near it behaves like a theremin.
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Old 29th Mar 2020, 11:07 pm   #28
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Default Re: Mullard 520 OP Transformer

Probably a lack of RF parasitic stoppers in the MC preamp.

David
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Old 30th Mar 2020, 7:28 am   #29
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Default Re: Mullard 520 OP Transformer

My first amp was far worse than that, mistakes are good, we learn from them. First off all wiring should be as short as poss , especially HT wiring as it can introduce stray inductance. Any AC htr wiring should AFAP be kept away from high gain, high impedance points like grids, and tucked in corners of the chassis if at all poss, if they have to cross another wire, do it at 90 degrees. Ground wiring again kept as short as poss, with no loops and with heavy current devices as near to the first PSU cap as poss, followed by less hungry stages with your low current IP stage at one end all brought back to a star ground.. Grid stopper resistor leads should be as short as poss on the valve base end

Solid wire looks neater but i don't like it as it breaks if moved around too much,which it will be whilst your building the amp, it's also possible to weaken it inadvertantly when stripping back. Try to follow a code, green for heaters, red for HT, black ground, yellow or orange for other HT type wiring like g2's. I use any old wiring mostly scavanged out of old washing machines, TV's, old PC SMPSU's, lots of pretty colours, preferably copper cored. Don't go using gurt big thick stuff where it isn't needed, most valve stages are only pulling a few mA, so thin wire here is fine. I never worry too much about wire being properly rated voltage wise, the exception being OPT primary wiring where V can exceed 1000v, albeit momenterally. Lastly, don't wrap wire round and round tags etc, half a turn is enough, chances are you'll have made a mistake and have to re-do the joint, nothing worse than having to wrestle half a mile of wire off a tag.

There's more, but that's the basics. Andy.
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Last edited by Diabolical Artificer; 30th Mar 2020 at 7:44 am.
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